1. Field of the Invention
The present invention relates to an apparatus and method for polishing an object material while estimating a removal amount of the object material using a model equation.
2. Description of the Related Art
An interlevel dielectric having a low dielectric constant is an essential technology for a high-density multi-level interconnect structure. This is because a smaller distance between layered metal interconnects results in a larger line-to-line capacitance, which causes a delay in signal transmission through the interconnects. Thus, there has recently been a trend to use a low-k material having a low dielectric constant as the interlevel dielectric. The low-k material has an advantage of having a low dielectric constant, but on the other hand, the low-k material has low mechanical strength and is relatively easily removed from a substrate. Thus, in order to prevent removal of the low-k material, a hard mask film may be formed on the low-k material.
FIG. 1 is a schematic view showing a part of a multi-level interconnect structure. As shown in FIG. 1, a hard mask film is formed on a low-k interlevel dielectric (hereinafter, this will be referred to as a low-k film). A barrier film is formed on the hard mask film, and a Cu film, which provides an interconnect metal, is further formed on the barrier film. These layered films form a multilayer structure, on which other multilayer structures are formed repeatedly. The multi-level interconnect structure is composed of a plurality of such multilayer structures at different levels.
When forming a new multilayer structure on the multilayer structure shown in FIG. 1, unwanted films are removed using a polishing apparatus. Since the hard mask film functions as a protective film for the low-k film, polishing should be stopped when the hard mask film remains with a certain thickness. Specifically, in FIG. 1, polishing is to be stopped after the barrier film is completely removed and before the hard mask is completely removed. Therefore, it is necessary to monitor a thickness of the hard mask film during polishing so as to accurately detect a polishing end point.
There are several techniques for monitoring a film thickness during polishing, such as a method using an optical sensor and a method using an eddy current sensor. However, the hard mask film is generally as thin as 50 nm to 60 nm, and this film is an oxide film. Consequently, it is difficult to accurately monitor a change in thickness of the hard mask film using these polishing end point detection techniques.